The present invention relates to multi-beam exposure units for multi-beam scanning, adapted to be used in multi-drum color printers and copiers, multicolor printers and copiers, single-color high-speed laser printers, single-color high-speed digital copiers, etc.
An image forming apparatus, such as a color printer or copier using a plurality of image forming units that include a photoconductor drum each, employs an exposure unit that can provide a plurality of light beams as many as a plurality of image data corresponding to color-separated color components, that is, as many as the image forming units at the least.
Exposure units of this type include a plurality of semiconductor laser devices, first lens group, deflector, second lens group, etc. The laser devices emit a predetermined number of light beams corresponding to the image data for the individual separated color components. The first lens group serves to reduce the cross section of each light beam from each laser device to a predetermined size and shape. The deflector deflects light beams, reduced to the predetermined size and shape by the first lens group, by continuously reflecting them in a direction perpendicular to the direction in which a recording medium, carrying thereon an image formed by means of each light beam, is transported. The second lens group serves to focus each light beam deflected by the deflector on a predetermined portion of the recording medium. In many cases, the direction in which each light beam is deflected by the deflector is regarded or referred to as a main-scanning direction, and the direction in which the recording medium is transported, that is, a direction perpendicular to the main-scanning direction, as a sub-scanning direction.
Some image forming apparatuses use a plurality of exposure units that correspond individually to the image forming units, while others use only one exposure unit or multi-beam exposure unit that can singly provide a plurality of light beams. These days, high-speed printers are proposed to ensure higher image forming speed and improved image resolution. In these printers, image data of the same color are exposed in parallel with one another so that high-resolution images can be formed at higher speed.
If the rotational frequency of reflecting surfaces of the deflector is expected to be increased to ensure high-speed, high-resolution image formation, in one such exposure unit, high-speed rotation of the reflecting surfaces requires use of an expensive bearing such as a pneumatic bearing. On the other hand, the motor speed has its upper limit, and a motor for high-speed rotation costs high, requiring an expensive driver circuit. The increase in the rotational frequency of the reflecting surfaces cannot cover this increase in cost. Moreover, increasing the rotating speed of the reflecting surfaces accelerates windage loss of the reflecting surfaces and entails louder windage noises.
In contrast with this, the increase of the motor speed can be restrained by increasing the reflecting surfaces in number. If this is done, however, the image frequency is augmented, so that noise components, which are probably superposed on image signals (image data), increase inevitably. The augmentation of the image frequency, however, imposes various restrictions on the control circuit design or mounting.
Accordingly, multi-beam exposure has already been proposed as a method for lowering the rotational frequency of the reflecting surfaces and the image frequency. In this method, a plurality of light beams are allotted individually to color-separated color components and deflected (for scanning) at a time. However, the multi-beam exposure involves the following various problems.
If U number of groups of light beams are used as light sources, corresponding to the number of the color-separated color components, and if V number of light beams are provided for each color component, the number of necessary semiconductor laser devices is U.times.V. Therefore, the number of light beams to be deflected by the deflector is U.times.V. In order to deflect these light beams collectively, U.times.V number of semiconductor lasers and first lenses and U.times.V-1 number of half-mirrors are needed to put together the U number of groups of light beams, each including V number of light beams. In consequence, the number of necessary components is increased, thus entailing higher cost.
In the case where a plurality of image forming units, e.g., W=4 in number in general, are provided corresponding individually to the color components to be used in a color copying apparatus or the like, moreover, the semiconductor lasers, first lenses, and half-mirrors in the aforesaid numbers must be provided in W number of sets. Accordingly, the size of the apparatus is also increased. In this case, furthermore, it is necessary to secure a space that is wide enough to hold all the elements.
On the other hand, the numbers of semiconductor lasers and first lenses can be reduced by using semiconductor laser arrays that include X number of light emitting points each. In this case, however, control of the space between each light beam and its adjacent ones or the next one to be deflected is complicated in a direction perpendicular to the direction in which the light beams are deflected by the deflector. If the space from each adjacent light beam or the next one to be deflected is changed slightly, the light beams are projected without being fully superposed in the direction perpendicular to the direction of deflection, so that jitters increase to lower the resolution.
The use of the semiconductor laser arrays that use X number of light-emitting points each allows some light beams that pass through lenses for converting the beams into a parallel or convergent beam to pass through any other portions of the lenses than the centers thereof. Therefore, aberrations for all the light beams can be corrected uniformly. In other words, it is difficult to equalize the aberrations for all the light beams by the same process of correction. Thus, the control for the space from each adjacent light beam or the next one to be deflected is restricted within a certain range.
Variation of the space from each adjacent light beam or the next one to be deflected results in uneven image density as well as increased jitters. This further causes the color copying machine to suffer unevenness of colors and color shifts.